1. Field of the Invention
The present invention is directed to an environmentally compliant applicator which uses a supersonic nozzle to convey high-speed triboelectrically charged particles to a substrate for effecting a coating or ablating the substrate. The supersonic nozzle of the applicator gun is fluid dynamically coupled to the substrate and the evacuator nozzle to enhance the triboelectric charging, reduce the inlet gas pressure required for supersonic expansion, and retrieve the excess particles. The impact energies achieved by accelerating small triboelectrically-charged projectile particles (i.e. &lt;100 microns in diameter) to speeds in excess of several hundred meters per second coupled with the electrostatic discharge energy provides an interaction energy that is significantly greater than that which can be attained in the art. Furthermore, the present invention is directed to a method for enhancing the melting and annealing of the substrate (e.g. to reduce residual stress) by triboelectric discharges which occur as a result of charge buildup on the substrate as triboelectrically charged particles impact the substrate.
The coating process or ablation process is determined by the material properties of the powder particles and substrate, and the energy densities deposited via the collisional energy and the triboelectric discharge energy at the impact zone. For producing metallized or polymerized coatings, the powder particles must be ductile or must have a sufficiently low glass transition temperature, and must adhere to the substrate for a sufficient duration to allow the binding energy between the particle and the substrate to establish a good bond.
The ablation process occurs with powder particles which have sufficient hardness and high melting point to preclude adhesion upon contact with the substrate. The ablation mechanism consists of erosion, melting, or evaporation of the uppermost substrate layer as determined by the speed of the particle, the triboelectric discharge energy, and the substrate material properties. The salient advantage of this invention over the art is the ablation efficiency provided by the high impact pressure of smaller diameter particles coupled with the triboelectric discharge energy.
2. Description of Related Art
Several references disclose methods of coating substrates by conveying powder particles in a carrier gas at high velocities and impacting the substrate to form the coating. Many of these references disclose use of high temperature carrier gases that melt the powder particles. U.S. Pat. No. 5,340,615 to Browning and U.S. Pat. No. 5,330,798 to Browning are exemplary of these high temperature methods.
Recently, U.S. Pat. No. 5,302,414 to Alkhimov, et al., discloses a low temperature method for coating substrates with metals, alloys, polymers, or mechanical mixtures of a metal and alloy by conveying powder particles at high speeds in a supersonic jet that is directed at a substrate to be coated.
Applications of the high temperature coating references are limited to depositions onto substrates which do not melt, burn, corrode, oxidize, or otherwise degrade in the high temperature environment of the carrier gas as it impinges on the substrate. Recently, U.S. Pat. No. 5,271,965 to Browning disclosed a method of injecting the particles at various locations in the expanding jet of the carrier gas so as to control the amount of particle melting prior to impact. The U.S. Pat. No. 5,302,414 to Alkhimov, et al., discloses a method of forming the gas and particles into a supersonic jet having a temperature sufficiently low to prevent thermal softening of the first material and a particle velocity from about 300 to about 1,200 m/sec. Although this latter method enables the coating of substrates without degrading materials by melting, burning, or evaporation, it does not address the issue of the residual stresses introduced in the coating and substrate materials by the plastic deformation of the impact process.
U.S. Pat. No. 4,979,680 to Bauch, et al., discloses a spray gun with electrokinetic charging of powdered material for the purpose of electrostatically coating workpieces with a plastic powder coating. U.S. Pat. No. 3,757,079 to Blomgren, Sr., uses an electrostatic field with corona discharge in the vicinity of an electric arc to provide a distortion-free weld of high integrity, and U.S. Pat. No. 3,895,211 to Pentegov, discloses a circuit design for implementing an electrostatic field generator. Triboelectric charging of particles during coating and painting processes have been conventionally used to affect coatings, the dispersion of particles to render more uniform coatings, and to control the coating thickness.
All of the electrostatic coating references disclose use of electrostatic charging of particles as a means of affecting coatings through electrostatic attraction or repulsion, but none of these references disclose the use of electrostatic discharge as a means of adding energy to the coating so as to affect the melting or annealing of the coating deposition. Only U.S. Pat. No. 3,757,079 to Blomgren, Sr., demonstrates that an electrostatic field with corona discharge may affect the integrity of an arc weld, but does not teach any device or method for coating or ablating a substrate.
Many types of sandblasting devices for cleaning the surface of a substrate have been disclosed in various patents. The following list is exemplary of the types of patents issued in this class: U.S. Pat. No. 5,283,985 to Browning, U.S. Pat. No. 5,160,547 to Kirschner, U.S. Pat. No. 3,894,364 to Korn, et al., U.S. Pat. No. 3,753,318 to Eskijian, U.S. Pat. No. 3,895,465 to Korn, et al., and U.S. Pat. No. 3,916,568 to Rose, et al. Two patents, U.S. Pat. No. 5,203,794 to Stratford, et al. and U.S. Pat. No. 4,703,590 to Westergaard also disclose methods of using particles which sublimate after impingement on the substrate.
All of these references use relatively large projectile particles (i.e. in excess of 100 microns in diameter) at low velocities (i.e. less than 300 meters/second) to abrade the substrate surface. The low impact pressures (determined by density of particle times velocity squared) attained by these methods abrade the substrate by an impact deformation process which fractures the brittle or work-hardened substrate surface. This abrasion process precludes the possibility of ablating the uppermost layer of many substrates by melting or evaporation, because the impact energies are too low.
In addition, several patents; U.S. Pat. No. 3,788,010 to Goff, U.S. Pat. No. 4,646,482 to Chitjian, U.S. Pat. No. 5,035,089 to Tillman, et al., U.S. Pat. No. 4,132,039 to Gilbert, et al., U.S. Pat. No. 3,894,851 to Gorman, U.S. Pat. No. 3,916,568 to Rose, et al., U.S. Pat. No. 5,197,160 to Smith A. L., U.S. Pat. No. 5,205,085 to Urakami, Fukashi, U.S. Pat. No. 5,256,201 to Gelain, et al., U.S. Pat. No. 5,263,897 to Kondo, et al., U.S. Pat. No. 5,269,949 Tuszko, et al., and U.S. Pat. No. 5,273,647 Tuszko, et al., disclose methods for recovering the excess blasting particles, retrieving, and filtering the abraded material. Although these devices have been effectively used with both subsonic and supersonic sandblasting nozzles, these references do not claim any special fluid dynamic coupling between the ejection nozzle and the recovery nozzle which maintains the supersonic two-phase flow within the supersonic nozzle nor do they disclose the use of a triboelectrical discharge as a means for affecting the coating or ablating process during the substrate impact.